There are several factors involved with external drive cases. There are lots of good ones.
3.5-in drive cases require their own power source. They can be had in 3 interface types: eSATA, USB, and Firewire. I find the eSATA and USB combo to be the most useful. Internally, they will be either SATA or PATA. There is no space for converter devices in an external case, but that can be rigged with the right cables.
2.5-in cases generally follow the same rules except most do not have their own power sources. Most will need some power other than the connector - this is done by a small 5 vdc source or a second USB link that has a power tip.
I have a pretty good supply of used HDDs, mostly PATA. My new systems are all SATA. I have all my digital imagery stored on externals - triple redundancy (that keeps the odds in my favor.)
My main system has a SATA RAID i array for data. That provides some redundancy, but then that array is backed up to an external drive as well. That adds to the robustness of the backup. RAID i is not really a backup, but does provide a safety net in the event of failure of one of the mirror drives.
For data transfer speed, my choices are, 1st - eSATA; 2nd - Firewire; and 3rd - USB.